Semiconductor diodes generally achieve rectification by means of asymmetric potential barriers created by carrier-depleted charge distributions (p-n junctions) or by electron affinity discontinuities at interfaces (Schottky barriers). The shape of these barriers cannot be controlled independently of charge carrier distributions, and cascading of a plurality of barriers is usually not possible. Recent work on diode structures has included rectification at n-n heterojunctions by A. Chandra et al., Electronics Letters, Vol. 15, page 90 (1979), control of current flow by a doping-produced potential hump in the bulk of a semiconductor material, J. M. Shannon, Applied Physics Letters, Vol. 35, page 63 (1979), and series connection of p-n diodes by degenerately doped tunnel junctions, M. Ilegems et al., Applied Physics Letters, Vol. 33, page 629 (1978).
In an early study of one of these device structures, the abrupt n-n heterojunction, W. G. Oldham et al. Solid State Electron, Vol. 6, page 121 (1963), proposed that unintentional compositional grading of the heterojunction resulted in the absence of rectification. Sixteen years later C. M. Garner et al., Journal of Applied Physics, Vol. 50, page 3383 (1979) reported the absence of rectification in n-n Al.sub.x Ga.sub.1-x As-GaAs heterojunctions even though the compositional grading was apparently not large enough to explain the result. In contrast, although Chandra et al., supra, purport to achieve room temperature rectification in n-n Al.sub.x Ga.sub.1-x As-GaAs, a report of their results at the January 1979 Conference on the Physics of Compound Semiconductor Interfaces, at Asilomar, Calif., raised considerable skepticism because, at room temperature, enough electrons should be thermally excited over the barrier to prevent rectification.
Thus, while prior work is somewhat contradictory on the impact which compositional grading in heterojunctions has on rectification, other work has exploited such grading to fabricate high voltage p-n junctions in Al.sub.x Ga.sub.1-x As-GaAs, Zh. I. Alferov et al., Soviet Physics Semiconductor, Vol. 1, page 1313 (1968), to facilitate making the p-side contact in Al.sub.x Ga.sub.1-x As double heterostructure lasers, Zh. I. Alferov et al., Soviet Physics Semiconductor, Vol. 2, page 1289 (1969), to achieve a short wavelength Al.sub.x Ga.sub.1-x As laser, W. Heywang, U.S. Pat. No. 3,965,347, issued June 22, 1976, and to produce a drift field for the collection of photoelectrons in a GaAs-Al.sub.x Ga.sub.1-x As solar cell, H. J. Hovel et al., U.S. Pat. No. 4,122,476, issued on Oct. 24, l978.